The principle of electrically controlled birefringence, the ECB effect or alternatively DAP effect (deformation of aligned phases), was described for the first time in 1971 (M. F. Schieckel and K. Fahrenschon, "Deformation of nematic liquid crystals with vertical orientation in electrical fields", Appl. Phys. Lett. 19 (1971), 3912). This was followed by papers by J. F. Kahn (Appl. Phys. Lett. 20 (1972), 1193) and G. Labrunie and J. Robert (J. Appl. Phys. 44 (1973), 4869).
The papers by J. Robert and F. Clerc (SID 80 Digest Techn. Papers (1980), 30), J. Duchene (Displays 7 (1986), 3) and H. Schad (SID 82 Digest Techn. Papers (1982), 244) have shown that liquid-crystalline phases must have high values for the ratio between the elastic constants K.sub.3 /K.sub.1, high values for the optical anisotropy .DELTA.n and values for the dielectric anisotropy .DELTA..epsilon. of from -0.5 to -5 in order to be suitable for high-information display elements based on the ECB effect. Electro-optical display elements based on the ECB effect have a homeotropic edge alignment.
Technical use of this effect in electro-optical display elements requires LC phases which must satisfy a multiplicity of requirements. Particularly important here are chemical resistance to moisture, air and physical effects, such as heat, radiation in the infrared, visible and ultraviolet regions and direct and alternating electric fields.
Technically suitable LC phases are furthermore required to have a liquid-crystalline mesophase in a suitable temperature range and low viscosity.
None of the series of compounds having a liquid-crystalline mesophase which have been disclosed hitherto includes a single compound which meets all these requirements. In general, therefore, mixtures of from two to 25, preferably from three to 18, compounds are prepared in order to obtain substances which can be used as LC phases. However, optimum phases cannot be prepared easily in this way, since no liquid-crystalline materials of significantly negative dielectric anisotropy and adequate long-term stability were hitherto available.
Matrix liquid-crystal displays (MLC displays) are known. Examples of nonlinear elements which can be used for individual switching of individual pixels are active elements (i.e. transistors). This is then referred to as an "active matrix", and a differentiation can be made between two types:
1. MOS (metal oxide semiconductor) transistors on silicon wafers as substrate. PA0 2. Thin-film transistors (TFTs) on a glass plate as substrate. PA0 a) a medium which additionally comprises one or more compounds of the formula II: ##STR6## PA0 b) A medium which additionally comprises one or more compounds of the formula III: ##STR7## PA0 in which PA0 c) A medium which essentially consists of four or more compounds selected from the formulae I1 and/or I2 and I3. PA0 d) A medium which comprises at least two compounds of the formula I3. PA0 e) A medium in which the proportion of compounds of the formula I1 and/or I2 in the mixture as a whole is at least 10% by weight. PA0 f) A medium in which the proportion of compounds of the formula I3 in the mixture as a whole is at least 30% by weight. PA0 g) A medium in which the proportion of compounds of the formula II in the mixture as a whole is at least 10% by weight. PA0 h) A medium in which the proportion of compounds of the formula III in the mixture as a whole is at least 5% by weight. PA0 i) A medium which comprises at least one compound selected from the formulae IIIa to IIIf: ##STR9## PA0 i) A medium which comprises at least one compound from the group consisting of the compounds IIIc1, IIIc2, IIId1-IIId4: ##STR10## PA0 j) A medium which essentially consists of: PA0 k) A medium which additionally comprises one or more compounds of the formulae IA and/or IB ##STR11## PA0 1) A medium which additionally comprises one or more compounds of the formulae IC to IG ##STR12## PA0 in which R.sup.13, R.sup.14, R.sup.15 and R.sup.16 are each, independently of one another, as defined for R.sup.11, and z and m are each, independently of one another, 1-6. R.sup.E is H, CH.sub.3, C.sub.2 H.sub.5 or n-C.sub.3 H.sub.7. PA0 m) A medium in which the compound of the formula I1 is selected from the group consisting of I1a to I1f: ##STR13## PA0 in which R.sup.11 and s are as defined above. R.sup.11 is preferably straight-chain alkyl having 1 to 6 carbon atoms, vinyl, 1E-alkenyl or 3E-alkenyl. PA0 n) A medium comprising one or more compounds of the formula I1a, in which R.sup.11 is straight-chain alkyl.
In the case of type 1, the electro-optical effect used is usually dynamic scattering or the guest-host effect. The use of single-crystal silicon as the substrate material limits the display size, since even modular assembly of various part-displays results in problems at the joints.
In the case of the more promising type 2, which is preferred, the electro-optical effect used is usually the TN effect.
A differentiation is made between two technologies: TFTs comprising compound semiconductors, such as, for example CdSe, or TFTs based on polycrystalline or amorphous silicon. Intensive work is being carried out worldwide on the latter technology.
The TFT matrix is applied to the inside of one glass plate of the display, while the other glass plate carries the transparent counterelectrode on the inside. Compared with the size of the pixel electrode, the TFT is very small and has virtually no adverse effect on the image. This technology can also be extended to fully colour-compatible image displays, where a mosaic of red, green and blue filters is arranged in such a way that each filter element is located opposite a switchable pixel.
The TFT displays disclosed hitherto usually operate as TN cells with crossed polarizers in transmission and are illuminated from the back.
The term MLC displays here covers any matrix display containing integrated nonlinear elements, i.e. in addition to the active matrix, also displays containing passive elements, such as varistors or diodes (MIM=metal-insulator-metal).
MLC displays of this type are particularly suitable for TV applications (for example pocket TV sets) or for high-information displays in automobile or aircraft construction. In addition to problems with respect to the angle dependence of the contrast and the response times, problems arise in MLC displays owing to inadequate resistivity of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris]. With decreasing resistance, the contrast of an MLC display drops. Since the resistivity of the liquid-crystal mixture generally drops over the life of an MLC display owing to interaction with the internal surfaces of the display, a high (initial) resistance is very important for displays which must have acceptable resistance values over a long service life.
The disadvantage of the MLC-TN displays disclosed hitherto is due to their comparatively low contrast, relatively high viewing-angle dependence and the difficulty of generating grey shades in these displays. EP 0 474 062 discloses MLC displays based on the ECB effect. The LC mixtures described therein are based on 2,3-difluorophenyl derivatives containing ester, ether or ethyl bridges and have low values of the "voltage holding ratio" (HR) after UV exposure. DE 44 26 798 discloses ECB mixtures which comprise compounds of the formula I1.
There thus continues to be a great demand for MLC displays having very high resistivity at the same time as a large working-temperature range, short response times and low threshold voltage which can be used to produce various grey shades.